1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for providing increased design flexibility for RF circuits, and more particularly for optimization of dielectric circuit board materials for improved performance in two port resonant lines.
2. Description of the Related Art
RF circuits, transmission lines and antenna elements are commonly manufactured on specially designed substrate boards. For the purposes of these types of circuits, it is important to maintain careful control over impedance characteristics and electrical length. If the impedances of different parts of the circuit do not match, this mismatch can result in inefficient power transfer, unnecessary heating of components, and other problems. Electrical length transmission lines and radiators in these circuits can also be a critical design factor.
Two critical factors affecting the performance of a substrate material are permittivity (sometimes called the relative permittivity or xcex5r) and the loss tangent (sometimes referred to as the dissipation factor). Another critical factor is the permeability (sometimes called the relative permeability or xcexcr). The relative permittivity and relative permeability determine the speed of the signal, and therefore the electrical length of transmission lines and other components implemented on the substrate. The loss tangent characterizes the amount of loss that occurs for signals traversing the substrate material. Accordingly, low loss materials become even more important with increasing frequency, particularly when designing receiver front ends and low noise amplifier circuits.
Printed transmission lines, passive circuits and radiating elements used in RF circuits can be formed in many different ways. Three common implementations are described below. One configuration known as microstrip, places the signal line on a board surface and provides a second conductive layer, commonly referred to as a ground plane. A second type of configuration known as buried microstrip is similar except that the signal line is covered with a dielectric superstrate material. In a third configuration known as stripline, the signal line is sandwiched between two electrically conductive (ground) planes. Ignoring loss, the characteristic impedance of a standard transmission line, such as stripline or microstrip, is equal to {square root over (Ll/Cl)} where Ll is the inductance per unit length and Cl is the capacitance per unit length. The values of Ll and Cl are generally determined by the physical geometry and spacing of the line structure as well as the permittivity and permeability of the substrate material(s) used to separate the transmission line structures. Conventional substrate materials typically have a relative permeability of approximately 1.0.
In conventional RF design, a substrate material is selected that has a relative permittivity value suitable for the design and the relative permeability typically approximately 1 for most common dielectric substrate materials. Once the substrate material is selected, the line characteristic impedance value is exclusively adjusted by controlling the line geometry and physical structure.
Radio frequency (RF) circuits are typically embodied in hybrid circuits in which a plurality of active and passive circuit components are mounted and connected together on a surface of an electrically insulating board substrate such as a ceramic substrate. The various components are generally interconnected by printed metallic conductors of copper, gold, or tantalum, for example that are transmission lines as stripline or microstrip or twin-line structures.
The permittivity and permeability of the chosen substrate material for a transmission line, passive RF device, or radiating element determines the physical wavelength of RF energy at a given frequency for that line structure. One problem encountered when designing microelectronic RF circuitry is the selection of a board substrate material that is optimized for all of the various passive components, and transmission line circuits to be formed on the board. In particular, the geometry of certain circuit elements may be physically large or miniaturized due to the unique electrical or impedance characteristics required for such elements. For example, many circuit elements or tuned circuits may need to be an electrical xc2xc wave. Similarly, the line widths required for exceptionally high or low characteristic impedance values can, in many instances, be too narrow or too wide respectively for practical implementation for a given substrate. Since the physical size of the microstrip or stripline is inversely related to the relative permittivity and permeability of the substrate material, the dimensions of a transmission line can be affected greatly by the choice of substrate board material.
Still, an optimal board substrate material design choice for some components may be inconsistent with the optimal board substrate material for other components, such as antenna elements or filters. Moreover, some design objectives for a circuit component may be inconsistent with one another. Accordingly, the constraints of a circuit board substrate having selected relative dielectric properties often results in design compromises that can negatively affect the electrical performance and/or physical characteristics of the overall circuit.
An inherent problem with the foregoing approach is that, at least with respect to the substrate material, the only control variable for line impedance is the relative permittivity, xcex5r. Changes in the relative permittivity affect Cl, the capacitance per unit length. This limitation highlights an important problem with conventional substrate materials, i.e. they fail to take advantage of the other material factor that determines characteristic impedance, namely the relative permability, xcexcr. Changes in the relative permeability affect Ll, the inductance per unit length of the transmission line.
Yet another problem that is encountered in RF circuit design is the optimization of circuit components for operation on different RF frequency bands. Line impedances and lengths that are optimized for a first RF frequency band may provide inferior performance when used for other bands, either due to impedance variations and/or variations in electrical length. Such limitations can reduce the effective operational frequency range for a given RF system.
Conventional circuit board substrates are generally formed by processes such as casting or spray coating which generally result in uniform substrate physical properties, including the permittivity. Accordingly, conventional dielectric substrate arrangements for RF circuits have proven to be a limitation in designing circuits that are optimal in regards to both electrical and physical size characteristics.
In a first embodiment according to the invention, a circuit for processing radio frequency signals comprises a substrate where the circuit can be placed. The substrate includes at least one substrate layer having a first set of substrate properties over a first region and at least a second set of substrate properties over a second region. The second set of substrate properties is different than the first set of substrate properties. The circuit further comprises at least one ground coupled to the substrate and a two port resonant line, at least a portion of said two port resonant line coupled to said second region.
In a second embodiment according to the invention, a printed circuit for processing radio frequency signals comprises a substrate upon which the circuit can be placed. The substrate includes at least one substrate layer having a first set of substrate properties over a first region and at least a second set of substrate properties over a second region. The second set of substrate properties provides a different dielectric permittivity and magnetic permeability as compared to the first set of dielectric properties. The printed circuit further comprises at least one ground disposed in or on said substrate and a lowpass filter. The lowpass filter includes a transformer line section on or within at least a portion of the first region and on or within at least a portion of the second region and at least a first stub section on or within the first region and on or within at least a portion of the second region. It should be understood within contemplation of the scope of the claims that xe2x80x9conxe2x80x9d may be also mean xe2x80x9cinxe2x80x9d or xe2x80x9cwithinxe2x80x9d in certain contexts. For example, a ground xe2x80x9conxe2x80x9d the substrate or a transformer line xe2x80x9conxe2x80x9d a first region should be understood to mean xe2x80x9con or in or withinxe2x80x9d the substrate or first region respectively.
In a third embodiment of the present invention, a printed circuit for processing radio frequency signals comprises a substrate including substrate regions upon which the printed circuit can be placed. The circuit is a lowpass filter including a transformer line section, at least a first stub section, and transmission line sections interconnecting the transformer line section with at least the first stub section. The transformer line section, the transmission line sections, and at least the first stub section are coupled to respective substrate regions that have substrate characteristics that are each independently customizable. The circuit further comprises at least one ground coupled to the substrate.